Toxicodendron radicans, better known as poison ivy (older synonyms are Rhus toxicodendron and Rhus radicans), is a poisonous North American plant that is well known for its production of urushiol, a clear liquid compound found within the sap of the plant that causes an itching rash in most people who touch it. The plant is not a true ivy (Hedera).
Poison ivy can be found growing in any of the following three forms:
as a trailing vine that is 10–25 centimeters tall (4 to 10 inches)
as a shrub up to 1.2 meters tall (4 feet)
as a climbing vine that grows on trees or some other support
Poison ivy grows throughout much of North America, including the Canadian Maritime provinces, Quebec, Ontario, Manitoba, and all U.S. states east of the Rockies, as well as in the mountainous areas of Mexico up to around 1,500 m (4,900 ft) (caquistle or caxuistle is the Nahuatl term), and is normally found in wooded areas, especially along edge areas. It also grows in exposed rocky areas and in open fields and disturbed areas. It also grows as a forest understory plant, although it is only somewhat shade tolerant. The plant is extremely common in suburban and exurban areas of New England, the Mid-Atlantic, and southeastern United States. Similar species, poison oak, and Toxicodendron rydbergii are found in western North America. Poison ivy rarely grows at altitudes above 1,500 m (4,900 ft), although the altitude limit varies in different locations. The plants can grow as a shrub up to about 1.2 metres (3.9 ft) tall, as a groundcover 10–25 cm (3.9–9.8 in) high, or as a climbing vine on various supports. Older vines on substantial supports send out lateral branches that may at first be mistaken for tree limbs.
The deciduous leaves of poison ivy are trifoliate with three almond-shaped leaflets. Leaf colour ranges from light green (usually the younger leaves) to dark green (mature leaves), turning bright red in fall; though other sources say leaves are reddish when expanding, turn green through maturity, then back to red, orange, or yellow in the fall. The leaflets of mature leaves are somewhat shiny. The leaflets are 3 to 12 cm long, rarely up to 30 cm. Each leaflet has a few or no teeth along its edge, and the leaf surface is smooth. Leaflet clusters are alternate on the vine, and the plant has no thorns. Vines growing on the trunk of a tree become firmly attached through numerous aerial rootlets. The vines develop adventitious roots, or the plant can spread from rhizomes or root crowns. The milky sap of poison ivy darkens after exposure to the air.
Poison ivy spreads either vegetatively or sexually. Poison ivy is dioecious; flowering occurs from May to July. The yellowish- or greenish-white flowers are typically inconspicuous and are located in clusters up to 8 cm above the leaves. The berry-like fruit, a drupe, mature by August to November with a grayish-white colour. Fruits are a favourite winter food of some birds and other animals. Seeds are spread mainly by animals and remain viable after passing through the digestive tract.
It is not particularly sensitive to soil moisture, although it does not grow in desert or arid conditions. It grows in a wide variety of soil types, and soil pH from 6.0 (acidic) to 7.9 (moderately alkaline). It can grow in areas subject to seasonal flooding or brackish water.
It is more common now than when Europeans first entered North America. Real estate development adjacent to wild, undeveloped land has engendered "edge effects," enabling poison ivy to form vast, lush colonies in such places. It is listed as a noxious weed in the U.S. states of Minnesota and Michigan and the Canadian province of Ontario.
Poison ivy and its relatives are virtually unknown in Europe.
The following three characteristics are sufficient to identify poison ivy in most situations: (a) clusters of three leaflets, (b) alternate leaf arrangement, and (c) lack of thorns. Although a number of other plants fit this simplified description, any plant with these criteria should be prudently avoided by people unfamiliar with identifying poison ivy. Identification by experienced people is often made difficult by leaf damage, leafless conditions during winter, and unusual growth forms due to environmental and/or genetic factors. The appearance of poison ivy can vary greatly between environments and even individual specimens within a single area.
Various mnemonic rhymes describe the characteristic appearance of poison ivy:
"Leaves of three, let it be."
"Hairy vine, no friend of mine." Poison ivy vines are very poisonous.
"Raggy rope, don't be a dope!" Poison ivy vines on trees have a furry "raggy" appearance. This rhyme warns tree climbers to be wary. Old, mature vines on tree trunks can be quite large and long, with the recognizable leaves obscured among the higher foliage of the tree.
"One, two, three? Don't touch me."
"Berries white, run in fright" and "Berries white, danger in sight."
"Longer middle stem, stay away from them." This refers to the middle leaflet having a notably longer stem than the two side leaflets and is a key to differentiating it from the similar-looking Rhus aromatica - Fragrant sumac.
"Red leaflets in the spring, it's a dangerous thing." This refers to the red appearance that new leaflets sometimes have in the spring. (Note that later, in the summer, the leaflets are green, making them more difficult to distinguish from other plants, while in autumn they can be reddish-orange.)
"Side leaflets like mittens, will itch like the dickens." This refers to the appearance of some, but not all, poison ivy leaves, where each of the two side leaflets has a small notch that makes the leaflet look like a mitten with a "thumb." (Note that this rhyme should not be misinterpreted to mean that only the side leaflets will cause itching, since actually all parts of the plant can cause itching.)
"If butterflies land there, don't put your hand there." This refers to the fact that some butterflies land on poison ivy, since they are not affected, which provides them protection as their predators avoid eating the plant.
"If it's got hair, it won't be fair." This refers to the hair that can be on the stem and leaves of poison ivy.
The reaction caused by poison ivy, urushiol-induced contact dermatitis, is an allergic reaction. Around 15% to 30% of people have no allergic response, but most people will become sensitized with repeated or more concentrated exposure to urushiol. Reactions can progress to anaphylaxis. Over 350,000 people are affected by poison ivy annually.
The pentadecylcatechols of the oleoresin within the sap of poison-ivy and related plants causes the allergic reaction; the plants produce a mixture of pentadecylcatechols, which collectively is called urushiol. After injury the sap leaks to the surface of the plant where the urushiol becomes a blackish lacquer after contact with oxygen.
Urushiol binds to the skin on contact, where it causes severe itching that develops into reddish coloured inflammation or non-coloured bumps, and then blistering. These lesions may be treated with Calamine lotion, Burow's solution compresses or baths to relieve discomfort, though recent studies have shown some traditional medicines to be ineffective. Over-the-counter products to ease itching—or simply oatmeal baths and baking soda—are now recommended by dermatologists for the treatment of poison ivy.
The oozing fluids released by scratching blisters do not spread the poison. The appearance of a spreading rash indicates that some areas received more of the poison and reacted sooner than other areas or that contamination is still occurring from contact with objects to which the original poison was spread. The blisters and oozing result from blood vessels that develop gaps and leak fluid through the skin; if the skin is cooled, the vessels constrict and leak less. If poison ivy is burned and the smoke then inhaled, this rash will appear on the lining of the lungs, causing extreme pain and possibly fatal respiratory difficulty. If poison ivy is eaten, the mucus lining of the mouth and digestive tract can be damaged. A poison ivy rash usually develops within a week of exposure and can last anywhere from one to four weeks, depending on severity and treatment. In rare cases, poison ivy reactions may require hospitalization.
Urushiol oil can remain active for several years, so handling dead leaves or vines can cause a reaction. In addition, oil transferred from the plant to other objects (such as pet fur) can cause the rash if it comes into contact with the skin. Clothing, tools, and other objects that have been exposed to the oil should be washed to prevent further transmission.
People who are sensitive to poison ivy can also experience a similar rash from mangoes. Mangoes are in the same family (Anacardiaceae) as poison ivy; the sap of the mango tree and skin of mangoes has a chemical compound similar to urushiol. A related allergenic compound is present in the raw shells of cashews. Similar reactions have been reported occasionally from contact with the related Fragrant Sumac (Rhus aromatica) or Japanese lacquer tree.